This invention relates to a method for manufacturing billets intended to be subsequently machined into a desired shape and dimensioned by plastic machining, such as rolling or forging. A capsule containing metal powder grains is heated to such a temperature as to insure the bonding together of the individual metal grains. The metal containing capsule is subjected to compressive forces of such a magnitude that all the voids within the capsule are pressed out to thereby effect a substantially homogeneous body having a high density. Any remaining porosity is eliminated during the subsequent forging or rolling operation, thus obtaining a material having such a theoretically high density.
The difficulty of achieving billets of a homogeneous composition and without segregations of voids at the upper portion of the ingot increases with increasing content of alloying materials when employing conventional pyrometallurgical methods. Ingot portions having segregations must be removed which results in the material gain decreasing with increasing alloying contents and an increased difficulty of achieving a homogeneous material with the desired composition. The poor output because of the high proportion of material that must be scrapped and the high price of the included alloying materials result in considerable cost and in an extreme increase in price of the finished material.
In an article entitled "The Consolidation of Metal Powders By Hot Working Within Sheaths" in Powder Metallurgy, 1958, Nos. 1, 2, pp. 94-103, J. Williams describes different methods of manufacturing products from powder grains. Billets or finished details may be produced directly by a number of different pressing methods. The powder is manufactured by breaking up a jet of molten metal. The metal droplets obtained are rapidly cooled and a favorable fine structure is obtained. This powder is enclosed in capsules and is machined at high pressure according to different forging or pressing methods into a solid body at a temperature which is so far below the melting temperature that undesirable structural changes through grain growth are avoided to the greatest possible extent. High-quality tool steel and superalloys have been manufactured commercially on a large scale by hot isostatic pressing of powder capsules which have been compressed in a pressure furnace and simultaneously sintered into a practically completely solid body. Both billets for rolling and forging and tools shaped to almost their desired end shape, have been manufactured. Powder-filled capsules have also been forged by tools or have been extruded. It is stated in the aforementioned article that forging in a closed tool does not result in a satisfactory product. Among other things, the capsule or sheath casing containing the metal powder becomes uneven or corrugated requiring removal of a substantial outer portion of the billet, resulting in a considerable loss of material. The problems are especially prominent in manufacturing a billet having a great height-to-diameter ratio. Thus, the method of pressing powder in capsules, as described in the aforementioned article, is inappropriate for manufacturing long billets suitable for subsequent rolling. Another drawback in connection with conventional pressing is that the powder nearest the wall of the capsule is cooled during insertion of the capsule into the press upon contact with the colder tool components which means that portions of the powder may be cooled to a temperature below the bonding temperature before it is possible to apply pressure. To a certain extent, such cooling may be counteracted by using hot tools. Heating of the tools, however, is disadvantageous since it reduces the stress values of the billet.